docs: Complete rewrite of plugin development documentation

- getting-started.mdx: Fixed package names (lokus-plugin-cli, lokus-plugin-sdk),
  correct import paths, accurate plugin class structure, real API examples
- api.mdx: Documented actual APIs (EditorAPI, UIAPI, CommandsAPI, FilesystemAPI,
  NetworkAPI, StorageAPI, ClipboardAPI) with correct method signatures
- cli.mdx: Updated all CLI commands with correct flags and options,
  fixed template names, correct registry URL (lokusmd.com)
- publishing.mdx: Fixed registry URL from registry.lokus.dev to lokusmd.com,
  updated token format and publishing flow

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: CodeWithInferno